As your therapist, what I would recommend is taking the initiative to invite yourself to your friends' gatherings when you see them. This doesn't justify their treating you like a second-class friend, and at some point it might be nice to be like "Y'know sometimes I feel like less of a friend when I get left out of stuff," but inviting yourself when you see your friends out might subliminally suggest to them that they should start inviting you to more things. I dunno. Maybe they feel like you wouldn't be interested in whatever it is they're doing.